In a piezoelectric actuator, a stack of piezoelectric plates is expanded or contracted by electric charging/discharging, thereby to rectilinearly move a piston or the like. A piezoelectric injector for fuel injection is opened and closed by such expansion and contraction of the stack of piezoelectric plates.
In addition, a fuel injection control apparatus (JP 2002-136156A), which controls the fuel injection into an internal combustion engine by driving the piezoelectric injector, includes a DC/DC converter of the boost type. In this converter, a voltage higher than a battery voltage is generated as a power source voltage across a capacitor, so that the piezoelectric actuator within the piezoelectric injector is charged by electric energy stored in the capacitor.
Here, an example of the fuel injection control apparatus of this type is shown in FIG. 13A.
First, a DC/DC converter 5 includes a boosting coil L0 which is fed with a battery voltage VB at one end thereof, a boost switch SW0 which is connected in series on the path between the other end of the boosting coil L0 and a ground potential (=0 V) being a reference potential, a reverse-flow preventing diode D0 whose anode is connected to the current path between the other end of the boosting coil L0 and the boost switch SW0, a capacitor C0 which is connected in series on the path between the cathode of the reverse-flow preventing diode D0 and the ground potential, and a boost controller 10 which controls the boost switch SW0.
In addition, the terminal of the boost switch SW0 opposite to the side of the boosting coil L0 and the terminal of the capacitor C0 opposite to the side of the diode D0 are connected to the ground potential through a resistor R0, which is for current detection. A switching element such as a MOSFET or a bipolar transistor is employed as the boost switch SW0.
In such a DC/DC converter 5 (U.S. Pat. No. 6,407,593, JP 2001-15332A), as shown in FIG. 13B, when the boost switch SW0 is turned on, a current Iro flows through the boosting coil L0 via the boost switch SW0 and the resistor R0. In addition, when the boost switch SW0 changes from ON to OFF, the capacitor C0 is charged through the reverse-flow preventing diode D0 by a counter-electromotive force induced in the boosting coil L0 (a high voltage which is several times to ten odd times as large as the battery voltage VB). Therefore, each time the ON/OFF operation of the boost switch SW0 is repeated, the capacitor C0 is charged, and the voltage VC of the capacitor C0 (capacitor voltage VC) is raised more.
Here, a boost SW current Iswo indicated by a solid line at the second stage of FIG. 13B is a current which flows through the boosting coil L0 and the boost switch SW0, and a capacitor current Ico indicated by a broken line is a charging current which flows from the charging coil L0 to the capacitor C0 through the reverse-flow preventing diode D0. In addition, both the boost SW current and the capacitor current become a current which flows through the resistor R0, as indicated by a dot-and-dash line at the third stage of FIG. 13B. Besides, discharging from the capacitor C0 onto the side of the boost switch SW0 is prevented by the reverse-flow preventing diode D0.
In the DC/DC converter 5, therefore, when the boost controller 10 starts boost switching control for turning on and off the boost switch SW0, this boost switch SW0 is first turned on.
In addition, as shown in FIG. 13B, when the boost controller 10 holds the boost switch SW0 in the ON state thereof it detects the current Iswo flowing through the boost switch SW0 (the boost SW current), on the basis of a voltage generated across the resistor R0. Besides, when the boost controller 10 determines that the current has increased to a preset OFF changeover threshold value Iofff, it changes the boost switch SW0 from ON to OFF. Further, when the boost controller 10 holds the boost switch SW0 in the OFF state thereof, it detects the charging current flowing from the reverse-flow preventing diode D0 to the capacitor C0 (the capacitor current), on the basis of a voltage generated across the resistor R0. In addition, when the boost controller 10 determines that the charging current has decreased to a preset ON changeover threshold value Ion (in this example, substantially 0 A), it changes the boost switch SW0 from OFF to ON.
Due to the repetition of such operations, the boost switch SW0 is repeatedly turned on and off, and the capacitor C0 is charged stepwise. In addition, the boost controller 10 monitors the capacitor voltage VC. When the capacitor voltage VC has reached a target value (a target charged voltage), the boost controller 10 stops the boost switching control and holds the boost switch SW0 in the OFF state thereof.
Further, as shown in FIG. 13A, the fuel injection control apparatus includes a charging/discharging coil 1 which is connected in series with a piezoelectric actuator 2, a charging path 6 which serves to feed a supply voltage from the positive terminal of the capacitor C0 (that is, the terminal of the capacitor C0 opposite to the ground potential side thereof) through a charging switch 4 to a series circuit 3 of the charging/discharging coil 1 and the piezoelectric actuator 2, and a discharging path 8 which is connected in parallel with the series circuit 3 and which serves to discharge the stored charges of the piezoelectric actuator 2 through a discharging switch 7.
The piezoelectric actuator 2 of this example is included in an injector which injects fuel into any one of the plurality of cylinders of the internal combustion engine, and it opens the injector by being expanded. This piezoelectric actuator 2 is one of a plurality of piezoelectric actuators which are connected in series with the charging/discharging coil 1 and in parallel with one another. Therefore, the terminal of the piezoelectric actuator 2 opposite to the side of the charging/discharging coil 1 (the terminal of the piezoelectric actuator 2 on the negative side thereof) is connected to the ground potential through a selection switch (cylinder selection switch) 9 which is a switching element for selecting this piezoelectric actuator 2 as an element to-be-driven. In addition, the selection switch 9 is turned on during a period for which the fuel injection into the cylinder corresponding to the piezoelectric actuator 2 is executed.
Still further, in this example, the charging switch 4 and the discharging switch 7 are MOSFETs, and diodes 4a and 7a in FIG. 13A are the parasitic diodes of the MOSFETs forming the switches 4 and 7, respectively.
In the fuel injection control apparatus, as shown in FIG. 14A, when a drive signal Sd for commanding the drive of the piezoelectric actuator 2 becomes an active level (in this example, “high”), a charging switching control in which the ON/OFF of the charging switch 4 is repeated is performed in the ON state of the selection switch 9 and the OFF state of the discharging switch 7, whereby the piezoelectric actuator 2 is charged and expanded to consequently open the injector.
That is, in the charging switching control mode (a charging period), the charging switch 4 is turned on in the OFF state of the discharging switch 7, whereby a charging current Ic is caused to flow from the positive terminal of the capacitor C0 to the piezoelectric actuator 2 as indicated by an arrow of solid line in FIG. 14A. Thereafter, the charging switch 4 is turned off, whereby a charging current (namely, a flywheel current) which flows due to energy stored in the charging/discharging coil 1 is caused to flow from the negative side to the positive side of the piezoelectric actuator 2 through the parasitic diode 7a of the discharging switch 7 as indicated by an arrow of broken line in FIG. 14A. The piezoelectric actuator 2 is charged stepwise by repeating such a procedure.
In FIG. 14C, a piezoelectric current Ip signifies the current which flows through the piezoelectric actuator 2, and a piezoelectric voltage Vp signifies the terminal voltage or positive side voltage of the piezoelectric actuator 2. Besides, in the waveform of the piezoelectric current in FIG. 14A, each of parts indicated by solid lines represents the charging current of the piezoelectric actuator 2 in the ON period of the charging switch 4, and each of parts indicated by broken lines represents the charging current of the piezoelectric actuator 2 in the OFF period of the charging switch 4.
In the fuel injection control apparatus, as shown in FIG. 14B, when the drive signal thereafter becomes an inactive level (in this example, “low”), a discharging switching control in which the ON/OFF of the discharging switch 7 is repeated in the OFF state of the charging switch 4 is performed, whereby the piezoelectric actuator 2 is discharged and contracted to consequently close the injector.
That is, in the discharging switching control mode (a discharging period), the discharging switch 7 is turned on in the OFF state of the charging switch 4, whereby a discharging current Id is caused to flow from the positive side of the piezoelectric actuator 2 through the charging/discharging coil 1 and the discharging switch 7 as indicated by an arrow of broken line in FIG. 14B. Thereafter, the discharging switch 7 is turned off, whereby a discharging current is caused to flow from the positive side of the piezoelectric actuator 2 to the positive terminal of the capacitor C0 through the charging/discharging coil 1 and the parasitic diode 4a of the charging switch 4 as indicated by an arrow of solid line in FIG. 14B, and the charges of the piezoelectric actuator 2 are recovered into the capacitor C0 by the discharging current. The piezoelectric actuator 2 is discharged stepwise by repeating such a procedure.
In the waveform of the piezoelectric current Ip in FIG. 14D, a negative direction corresponds to a discharging direction. Besides, in the waveform of the piezoelectric current Ip, each of parts indicated by broken lines represents the discharging current of the piezoelectric actuator 2 in the ON period of the discharging switch 7, and each of parts indicated by solid lines represents the discharging current of the piezoelectric actuator 2 in the OFF period of the discharging switch 7.
Meanwhile, in the fuel injection control apparatus of this type, when the charging switch 4 has been turned on in the charging switching control, the current (the current in a discharging direction) flows through the capacitor C0, as indicated by the arrow of the solid line in FIG. 14A, and also when the discharging switch 7 has been turned off in the discharging switching control, the current (the current in a charging direction) flows through the capacitor C0, as indicated by the arrow of the solid line in FIG. 14B.
In the DC/DC converter 5 disposed in the prior art fuel injection control apparatus, the charging current which flows from the reverse-flow preventing diode D0 to the capacitor C0 is detected by the resistor R0, and the boost switch SW0 is controlled using the detection value.
In consequence, while the charging switching control and the discharging switching control for driving the piezoelectric actuator 2 are being executed, the boost switching control in the DC/DC converter 5 cannot be properly carried out. This is for the reason that, when the boost switching control is carried out during the performances of the charging switching control and the discharging switching control, the current detected by the resistor R0 is not always the charging current from the reverse-flow preventing diode D0 to the capacitor C0.
In the prior art fuel injection control apparatus, therefore, as shown in FIG. 15, the boost operation of the DC/DC converter 5 (that is, the performance of the boost switching control) is stopped during the performances of the charging switching control and the discharging switching control for driving the piezoelectric actuator 2.
However, in a case where the drive interval Tinj of the piezoelectric actuator 2 (that is, interval of no fuel injection) is shortened, the voltage Vp of the capacitor C0 will fail to rise up to the target value necessary for the drive of the piezoelectric actuator 2, by the next drive timing, and the charged energy Ec of the capacitor C0 might become insufficient by an amount Ei. In addition, when the charged energy of the capacitor C0 becomes insufficient, a response rate at the expansion of the piezoelectric actuator 2 lowers, and in turn, the opening response rate of the injector lowers to incur the worsening of control precision.
Especially in the diesel engine of a vehicle, a diesel common rail system (CRS) wherein fuel of high pressure accumulated in a common rail is injected from the injector is known. In the CRS, multistage injection wherein fuel is injected into one cylinder a plurality of times in a divided manner is executed, thereby to attain the improvement of emission, the reduction of noise and the improvement of fuel consumption. With the prior art technology, however, the charging of the capacitor C0 becomes too late in performing the multistage injection at which the discharging frequency of the capacitor C0 becomes high, and the injector cannot be precisely driven.